1. Field of the Invention
The present invention relates to an elastic roller for use in an image forming apparatus, and more particularly to an elastic roller capable of performing liquid squeezing and absorbing functions at the same time and adapted for use in a device involving a liquid such a liquid developer or a cleaning liquid in an image forming process.
In the field of image forming technologies such as electrophotography, electrostatic printing and electrostatic recording, there are already known various processes for developing, namely rendering visible, an electric latent image formed on a latent image carrier such as a so-called photosensitive member utilizing a photoconductive material or an electrostatic recording material. Such image visualization or development is generally achieved by applying particles of an electroscopic material or developer (generally called toner) selectively attracted or repelled by the electrostatic charge of said latent image onto a surface carrying said latent image thereon, although the details may differ according to whether a direct development or a reversal development is desired. In case of direct development the developer particles are deposited in the areas of latent image while in case of reversal development the developer particles are deposited outside said areas.
Among such developing processes, there are well known a dry development utilizing a developer consisting of dry powder and a wet development utilizing a liquid developer in which toner particles are dispersed in a liquid. Particularly, the latter liquid development is frequently employed in a simpler image forming apparatus because of the relatively simple mechanism required in comparison with the mechanism required for dry development and also because liquid development involves fewer drawbacks such as scattering of developer. In summary said liquid development comprises bringing a so-called liquid developer, including toner particles dispersed in an insulating carrier liquid of a volume resistivity higher than 10.sup.10 .OMEGA.cm, and of a dielectric constant smaller than 3, for example a paraffinic hydrocarbon, into contact with a latent image holding surface whereby said toner particles in said carrier liquid are attracted by and deposited on the electrostatic latent image on said surface to render said latent image visible. In apparatus utilizing such liquid development the use of elastic rollers has been quite common in the developing section, squeezing section, cleaning section etc., thereof.
2. Description of the Prior Art
As an example of such elastic roller applicable for the above-mentioned purposes, there is disclosed in the Japanese Laid-Open Patent Sho-52/55644 an elastic roller consisting of a sponge layer containing a continuous liquid-holding space therein, periphery of which is covered for example with a flexible liquid-permeating net. Such roller is capable of providing a desirable nip width in rotation under pressure contact with a rigid surface of a photosensitive element or an insulating element, and of squeezing out a liquid contained therein or absorbing a liquid according to the elastic deformation thereof. Such roller, therefore, is considered well suited for the liquid-handling purposes, for example in the developing or cleaning section.
However, such roller provided with a net-shaped external layer on the surface thereof, probably due to a difference in flexibility or hardness between the internal and external layers, tends to develop wrinkles in said external layer or net during the use of such roller, namely upon pressure contact with a rigid surface. Such wrinkle formation has proved to be particularly marked in the end portions of said roller. Also the structure of said net at the roller end is important. If the net end is exposed on the roll end, fibers of said net become gradually loose from the end portion of net to deteriorate the proper function of the roller and to render impossible uniform squeezing and absorbing effect over the entire width of the roller. Such an elastic roller, when employed in the developing step, will result in an uneven development according to the surfacial wrinkles or to the uneven pressure along the axis thereof, and, when employed for removing excessive liquid developer, will result in uneven liquid squeezing. Further, as a cleaning roller it will result in uneven cleaning which in turn leads to uneven charging in the succeeding electrostatic charging step.